1. Field of the Invention
This invention relates to a humidity-sensitive element using a polyimide film as a humidity-sensitive film.
2. Description of the Related Art
A humidity sensor converts a change resulting from humidity and moisture to an electric quantity and executes signal processing by use of an electronic circuit. It has been widely employed to satisfy a demand for humidity control in a variety of objects. Among humidity sensors, a polymer humidity sensor that utilizes a change of an electric characteristic of a polymer material, such as an electric resistance or a dielectric constant, with absorption and desorption of water, is known. An electrostatic capacitance type, as one such sensor utilizes the phenomenon that a composite dielectric constant of a polymer increases when moisture is absorbed, and detects this phenomenon as a capacitance change between electrodes.
A large number of humidity sensors using polyimide, as a humidity-sensitive film in a humidity sensitive element for a humidity sensor, have been reported in the past. However, these humidity sensors have a water absorption ratio as great as 1 to 3% and involve the problem of a drift of quality when they are left standing at a high temperature and a high humidity. This drift of quality presumably results from the increase of the water absorption ratio due to hydrolysis and swelling of the polyimide. To reduce the drift, therefore, a method for reducing the water absorption ratio has been proposed (for example, refer to Japanese Patent No. 2,529,136).
This method suppresses the increase of the water absorption ratio by adding fluorine and increasing the hydrophobicity. However, because of the high hydrophobicity resulting from fluorine, the water absorption ratio itself drops and the drop in sensitivity becomes remarkable. A biphenyltetracarboxylic acid type polyimide (for example, U-varnish S or A, product of Ube Kosan K. K.) is known as a polyimide having a low water absorption ratio. Because this polyimide has a property such that it undergoes laminar orientation, from the aspect of a molecular structure, water absorption among the molecules can be suppressed. When experiments are carried out by using this polyimide for the humidity sensitive film, the tendency that the drift under a high temperature high humidity condition is small at the initial stage, and the effect of the low water absorption ratio, can be observed but, after a while, the drift gradually takes place. The reason is presumably because the clearances among the molecules are expanded due to swelling under a high temperature high humidity condition with the result of an increase of the water absorption ratio. Therefore, the present inventor has previously proposed a molecular structure for forming a network structure by coupling the polyimide molecules with one another to suppress the volume change due to swelling that occurs when the absorped water undergoes aggregation (Japanese Unexamined Patent Publication (Kokai) No. 2003-232765).
However, this humidity sensitive element cannot sufficiently suppress the drift when it is left standing in a high temperature high humidity atmosphere of 65° C. and 90% RH, for example. In the film formation process of the polyimide of this humidity sensitive element, a polyamide acid, the terminals of which are terminated with acetylene, is heat-treated. Therefore, three acetylenes at the terminals react with one another during hardening and form a benzene ring, and the network structure is thus formed. Because the imidation reaction due to dehydration and ring closure of the polyamide acid occur simultaneously at this time, the reaction ratio of the formation of the benzene rings is presumably about 30%. Therefore, the network structure cannot be formed in a sufficient density and the effect of suppressing the drift is believed to be small.
The invention solves the problems described above by using a polyimide having a molecular structure capable of forming a network structure at a high density as a humidity sensitive film, and provides a humidity sensitive element having a small drift after being left standing at a high temperature and a high humidity but excellent characteristics.